How To Find The Exact Conception Date I Need Signs To Know That My Dog Is Ready To Deliver?

I need signs to know that my dog is ready to deliver? - how to find the exact conception date

I have a Labrador (Sammy), who is 4 years old and pregnant. I do not know the exact date has designed them and is her first litter. This is for both of us and I have done much research on what to expect new, but I'm afraid they go to work and I will not be there to ensure her and the puppies are good. It was a B-Day gift to me by my friend I feel very bad if it is only through this and something happened. That's what happened, what I see, they will get plenty of rest, start your breasts will sag (which perhaps) a sign of unity, that some sort of activity is replaced, it must then rest and it was eating more. One question that I have not found an answer to: How can we know the moment of conception, when hanging breasts, how to start mHe ran before work begins? Any help in this matter would be very grateful. She saw the vet and is healthy, but can not be estimated without the day of conception, an the due date.
Thank you again!